126
9 Materials Near the Layered Boundary
composite material. Then, upon completing the phase separation, the materials made
in air would have separate crystallites of each phase such that the more ordered phase
would then cycle nicely explaining the higher capacities obtained in air when regular
cooled even though phase separation was more evident in the XRD.
Figures 9.6 and 9.7 show the voltage vs. capacity and dQ/dV vs. voltage plots for
these cells. The main feature in the dQ/dV plots was a peak near 3.75 V with some
smaller peaks near 4.5 V corresponding to a small high voltage plateau in the capacity
versus voltage plots. The most relevant features in the cells made from regular cooled
A 9 materials heated in 2 % oxygen were a large irreversible capacity when cycled up
to 4.8 V and a significant impedance growth in both regular cooled materials. This
impedance can be seen as a voltage difference between charge and discharge, which
was noticeably larger in the regular cooled samples as compared to the quenched
samples. This change was also seen in the samples made in air, though it was less
severe, while the cells made from sample B 9 showed no such impedance growth such
that this can again be attributed to the presence of multiple phases appearing during
cooling.
9.6 Li 1+x [Ni 0.5 Mn 0.5 ] 1−x O 2 Series with 0 ≤ x ≤ 0.24
Figure 9.8 shows the region of the XRD patterns near 44
◦ for the composition line
Li 1+x [Ni 0.5 Mn 0.5 ] 1−x O 2 with x = 0.0, 0.04, and 0.08 made at 900
◦ C in air and regular
cooled. The samples have not been analyzed by elemental analysis, but based on the
small amount of lithium loss in the previous samples, these samples lie near points
A 9 , B 9 , and C 9 in Fig. 9.1. The sample with x = 0.00 (point A 9 ) shows evidence for
the presence of a second phase, consistent with the XRD pattern expected for sample
A 9 heated to 900
◦ C in air and regular cooled in Fig. 9.1. The samples with x = 0.04
and 0.08 do not show evidence of a second phase, although the peak width increases
with x. The crystallite size and microstrain may therefore depend on the composition,
x. Table 9.2 summarizes the XRD results for the Li 1+x [Ni 0.5 Mn 0.5 ] 1−x O 2 series with
0 ≤ x ≤ 0.24. The XRD patterns therefore confirm that changing the value of x
in this manner moved samples from outside the single-phase region (sample A 9 ,
x = 0.0) to within the single-phase region (samples B 9 and C 9 ), and then outside
the single-phase region, where excess Li 2 CO 3 was observed in the XRD patterns
(x = 0.20 and 0.24). Studies of these samples also allow the determination of the
impact of the layered–layered phase separation as all samples were made under
identical conditions.
Figure 9.9 shows the reversible specific capacity of the Li 1+x [Ni 0.5 Mn 0.5 ] 1−x O 2
samples during the first and fourth discharges. This figure shows that samples should
be prepared as single-phase materials in order to deliver the highest capacity and
that phase separation into layered–layered composites (x = 0 in Fig. 9.9) should be
avoided. Figure 9.9 also shows that samples closest to the single-phase boundary
have the highest capacity when cycled to 4.4 V. This illustrates the importance of a
complete understanding of the phase diagram in order to avoid Ni
3+ in the starting
material and to produce materials with the best electrochemical performance.
9 Materials Near the Layered Boundary
composite material. Then, upon completing the phase separation, the materials made
in air would have separate crystallites of each phase such that the more ordered phase
would then cycle nicely explaining the higher capacities obtained in air when regular
cooled even though phase separation was more evident in the XRD.
Figures 9.6 and 9.7 show the voltage vs. capacity and dQ/dV vs. voltage plots for
these cells. The main feature in the dQ/dV plots was a peak near 3.75 V with some
smaller peaks near 4.5 V corresponding to a small high voltage plateau in the capacity
versus voltage plots. The most relevant features in the cells made from regular cooled
A 9 materials heated in 2 % oxygen were a large irreversible capacity when cycled up
to 4.8 V and a significant impedance growth in both regular cooled materials. This
impedance can be seen as a voltage difference between charge and discharge, which
was noticeably larger in the regular cooled samples as compared to the quenched
samples. This change was also seen in the samples made in air, though it was less
severe, while the cells made from sample B 9 showed no such impedance growth such
that this can again be attributed to the presence of multiple phases appearing during
cooling.
9.6 Li 1+x [Ni 0.5 Mn 0.5 ] 1−x O 2 Series with 0 ≤ x ≤ 0.24
Figure 9.8 shows the region of the XRD patterns near 44
◦ for the composition line
Li 1+x [Ni 0.5 Mn 0.5 ] 1−x O 2 with x = 0.0, 0.04, and 0.08 made at 900
◦ C in air and regular
cooled. The samples have not been analyzed by elemental analysis, but based on the
small amount of lithium loss in the previous samples, these samples lie near points
A 9 , B 9 , and C 9 in Fig. 9.1. The sample with x = 0.00 (point A 9 ) shows evidence for
the presence of a second phase, consistent with the XRD pattern expected for sample
A 9 heated to 900
◦ C in air and regular cooled in Fig. 9.1. The samples with x = 0.04
and 0.08 do not show evidence of a second phase, although the peak width increases
with x. The crystallite size and microstrain may therefore depend on the composition,
x. Table 9.2 summarizes the XRD results for the Li 1+x [Ni 0.5 Mn 0.5 ] 1−x O 2 series with
0 ≤ x ≤ 0.24. The XRD patterns therefore confirm that changing the value of x
in this manner moved samples from outside the single-phase region (sample A 9 ,
x = 0.0) to within the single-phase region (samples B 9 and C 9 ), and then outside
the single-phase region, where excess Li 2 CO 3 was observed in the XRD patterns
(x = 0.20 and 0.24). Studies of these samples also allow the determination of the
impact of the layered–layered phase separation as all samples were made under
identical conditions.
Figure 9.9 shows the reversible specific capacity of the Li 1+x [Ni 0.5 Mn 0.5 ] 1−x O 2
samples during the first and fourth discharges. This figure shows that samples should
be prepared as single-phase materials in order to deliver the highest capacity and
that phase separation into layered–layered composites (x = 0 in Fig. 9.9) should be
avoided. Figure 9.9 also shows that samples closest to the single-phase boundary
have the highest capacity when cycled to 4.4 V. This illustrates the importance of a
complete understanding of the phase diagram in order to avoid Ni
3+ in the starting
material and to produce materials with the best electrochemical performance.